This invention relates to improvements in methods of effecting chemical reactions and is particularly suitable for effecting accelerated chemical reactions in a crosscurrent flow, nested tube apparatus wherein the influent fluid is in heat transfer relation with the effluent. The process of this invention is also particularly, although not exclusively, suitable for effecting chemical reactions in a deep well reaction apparatus, such as used for carrying out the wet oxidation of sewage sludge and other liquid waste streams.
As set forth more fully in U.S. Pat. No. 4,272,383 of McGrew, assigned to the assignee of the present application, there are a number of chemical reactions that may be accelerated at elevated temperatures and pressures. Prior to the installation of a deep well reaction apparatus by the assignee at Longmont, Colo., based upon the concepts disclosed in the above-identified patent of McGrew, the methods used to effect wet oxidation of sewage sludge, for example, involved placing the waste in a high temperature, high pressure reactor at substantially ground surface level. Air or oxygen is pumped into the reaction vessel and heat is externally applied as disclosed, for example, in U.S. Pat. No. 2,665,249 of Zimmermann.
More recently, but prior to the above-identified McGrew patent, the prior art proposed several methods for effecting wet oxidation of liquid waste streams in a deep well reaction apparatus having concentric conduits extending vertically below ground in a subterranean shaft. U.S. Pat. No. 3,449,247 of Bauer, for example, discloses a method for effecting wet oxidation of a mixture of combustible refuse and fluid sewage carried out in the lower portion of a shaft extending into the earth a sufficient distance to provide the desired pressure by the head of fluid material in the shaft. The disclosed apparatus includes vertically-extending concentric conduits wherein the water and reactants comprising the influent stream are flowed downwardly in the outer conduit and the water and reaction product are flowed upwardly through the center conduit in heat transfer relation with the influent. Air or oxygen-enriched air is injected in the influent liquid and the disclosed method further includes injecting air in the reaction zone at the lower extent of the conduits. U.S. Pat. No. 3,606,999 of Lawless discloses a method and deep well reaction apparatus for treatment of fluid streams, including wet oxidation of sewage, which includes a vapor trap or separation device for collecting the gaseous reactant of the reaction. U.S. Pat. No. 3,853,759 discloses a pyrolytic method for treating a liquid sewage stream which includes limiting the combustion of the material by restricting the process to the oxygen present in the material, whereby the pressure at the bottom of the influent column supposedly causes the heated material to rise in the effluent column. Finally, U.S. Pat. No. 4,564,458 of Burleson, which is subsequent to the McGrew patent, discloses a method of effecting wet oxidation of organic waste materials in a waste water stream in a deep well reaction apparatus at super-critical temperatures and pressures, wherein the wet oxidation reaction is to be initiated by flowing an electric current through the water at the bottom of the well to generate resistance heat. It should be understood, however, that these prior art disclosures, excluding the McGrew patent, are purely theoretical and generally impractical. The disclosed deep well reaction apparatus were not built or tested.
An experimental deep well reaction apparatus was designed, installed and successfully operated by the assignee of the present application at Longmont, Colo. This installation, however, established that certain parameters and principles disclosed in the McGrew patent are not accurate or practical. For example, the McGrew patent states that no pumping pressure will be required to sustain a continuous flow through the deep well reaction apparatus and that there is no pressure differential across the wall dividing the concentric tubes or pipes. In fact, as established by the actual installation at Longmont, Colo., a substantial pumping pressure is required during start-up and continued operation of a deep well reaction apparatus. More importantly, the required pumping pressure increases during operation because of plugging of the downcomer and the build-up and accumulation of organic and inorganic scale or fouling. In actual practice, it was necessary to periodically shut down the operation of the deep well reaction apparatus to remove the plug and clean the conduits when the pumping pressure exceeded a predetermined maximum pressure, requiring frequent downtime. The initial pumping pressure averaged between 400 and 500 psi when the conduits were cleaned because of the frictional pressure drop. However, the pumping pressure increased to about 600 psi, because of the accumulation of plugging of the downcomer and fouling, requiring shutdown and cleaning of the conduits. As will be understood, the pumping of the influent liquid into the downcomer conduit at the pressures defined and the frequent shutdowns resulted in substantial expenses. A primary object of the present invention is to reduce these costs while improving the efficiency of the process.
It was further believed by those skilled in the art that any boiling of the liquid in the conduits could result in "geysering" if the boiling continued for any substantial period of time. The conduits extended into the earth over 5000 feet. Thus, the combined lengths of the upcomer and downcomer conduits totals about two miles. If geysering occurred as a result of boiling, the resultant geyser of steam, reaction products and sewage, for example, could cause injury and substantial damage to the deep well reaction apparatus. The McGrew patent, therefore, emphasizes the importance of controlling the temperature of the liquid in the reaction zone by adding or removing heat to accomplish a maximum reaction rate with the vapor pressure of the influent fluid at the local temperature being maintained always lower than the local pressure to prevent boiling of the influent fluid.
It has now been discovered that boiling of the effluent fluid results in unexpected improvements in the efficiency of the operation of a deep well reaction apparatus, including reduced pumping pressures, increased mass flow rates, reduced downtime for cleaning and decreased start-up time. It is therefore a further object of the present invention to initiate boiling of the effluent liquid in the upcomer conduit to reduce the pumping pressure and improve the efficiency of the reaction apparatus.